Copy guard techniques such as the serial copy management system (SCMS) and copy generation management system (CGMS) are used to prevent illegal acts (such as copyright infringement acts) of copying information from one information recording medium to another information recording medium. For example, two bits of copyright protection information are recorded on an information recording medium, ‘00’ indicating that copying is possible, ‘10’ indicating that copying is possible only once, ‘11’ indicating that copying is prohibited. The information recording apparatus abides by the copyright protection information recorded on the information recording medium. The copyright protection information recorded on an information recording medium can be tampered with, however. Therefore, a method has been proposed that, in addition to the two bits of copyright protection information, adds the same information as given by the two bits of copyright protection information as a watermark to the digital data obtained by encoding of video information, detects both during playback, and decides whether to output the data from the decoder or to disable output according to the result of a comparison of the two (see, for example, Patent Document 1).
Another technique that is in general use for protecting copyrights on digital content is encryption of the content. In this copyright protection technique, the data to be protected are encrypted when the data are recorded, and the encryption key needed for decrypting the encrypted data and reproduction control data for copyright protection are embedded in the encrypted data as an electronic watermark. When the data are reproduced, the electronic watermark is detected, the encryption key needed for decrypting the encrypted data and reproduction control data for copyright protection are recovered, the encrypted data are decrypted by use of the recovered encryption key, and the reproduction of the copyright-protected data is controlled by the recovered reproduction control data (see, for example, Patent Document 2).    Patent Document 1: Japanese Patent Application Publication No. 2006-87096    Patent Document 2: Japanese Patent Application Publication No. 2003-195759